In a solar cell in which incident light is converted to electricity by a photoelectric conversion unit, if the light reflected from the surface is large, the light entering the solar cell decreases, and the obtainable power thus decreases. Therefore, to increase the photoelectric conversion efficiency of the solar cell, it is essential to reduce the optical reflectance on the surface so that more light is taken in. An effective way to reduce the optical reflectance is, for example, to provide an antireflection film on the surface of the solar cell.
However, even if an antireflection film is used, a loss still occurs due to reflection of several percent of the incident light. Therefore, minute irregularities referred to as “texture” are further formed on the surface of the solar cell so that the reflectance is reduced due to an optical confinement effect.
For example, single-crystal silicon can be easily formed into a random pyramid-shaped irregular structure, as the texture, by using an alkaline solution to perform anisotropic etching. The optical reflectance on the surface of the solar cell can be considerably reduced by using these methods. However, a method of realizing a further reduction in the reflectance is already known. In this method, the texture structure has regularly arranged pyramid-shaped openings. A manufacturing method of the pyramid-shaped openings is already known in which an etching mask is formed on the surface of single-crystal silicon and the mask is then used to perform anisotropic etching. By regularly providing the pyramid-shaped texture on a light-receiving surface in this manner, the optical confinement effect due to multiple scattering can be increased further when compared with a random texture.
In a conventional solar cell that uses a crystalline silicon substrate, such as a single-crystal silicon substrate, a pyramid-shaped irregular shape (texture) due to the surface of (111) is formed on the surface of the silicon (100) substrate by performing anisotropic etching.
Furthermore, a heterojunction solar cell technique in which an amorphous silicon layer or a microcrystal silicon layer is laminated on a single-crystal silicon substrate to form a PN junction has been disclosed (Patent Literature 1). The problem is that with such a hetero-structure many defects occur at the heterojunction interface, and high conversion efficiency cannot be achieved. However, a solar cell technique having a structure has been disclosed in which a thin genuine amorphous silicon layer is sandwiched between a single-crystal silicon substrate and an amorphous silicon substrate to reduce the number of defects at the heterojunction interface (Patent Literature 2).
In the solar cell structure described above, because of the steep pyramid-shaped irregular shape formed on the silicon substrate, defects may occur in the amorphous silicon layer formed on the substrate, or the film thickness may become non-uniform, thereby resulting in a decrease in output characteristics. Patent Literature 3 discloses a technique in which isotropic etching is performed on a substrate that is provided with pyramid-shaped irregularities so that a rounded valley portion is formed, thereby solving the problems described above.